1. Field of the Invention
The invention concerns pressure-sensitive adhesive tape, an adhesive layer of which either is foamlike because it includes glass microbubbles as taught in U.S. Pat. No. 4,223,067 or comprises a foam because it has a cellular core as taught in U.S. Pat. No. 4,415,615.
2. Description of the Related Art
Foam-backed pressure-sensitive adhesive tape is commonly used to adhere an article to a substrate. The foam backings of such tapes often are pigmented with carbon black to afford a dark appearance that camouflages their presence.
The pressure-sensitive adhesive tape of the above-cited U.S. Pat. No 4,223,067 has a foamlike appearance and character, even though it is not a foam, and is useful for purposes previously requiring a foam-backed pressure-sensitive adhesive tape. A foamlike tape of this type now on the market is made by ultraviolet light polymerization of a layer of an adhesive-forming mixture containing colorless glass microbubbles, which act as a white pigment that makes the tape undesirably visible in uses such as sealing a skylight or attaching body side moldings to automotive vehicles or simulated mullion bars to glass windows. Because of its superior performance characteristics, this foamlike tape is often preferred to foam-backed tapes for such purposes and would be more acceptable if its adhesive layer were sufficiently dark to camouflage the tape. If carbon black or other pigment were added to the photopolymerizable adhesive-forming mixture in amounts sufficient to produce a desirably dark appearance, this would block the ultraviolet radiation from polymerizing the mixture to a pressure-sensitive adhesive state. Up to about 0.06 percent by weight of carbon black can be employed without undue interference with the polymerization of a 1.0 -mm layer (as evidenced by adhesive and cohesive values), but such small amounts produce only a pastel grey color that would be undesirably noticeable for uses such as those mentioned above. When other pigments are substituted for the carbon black, they also cannot be used in amounts producing a color deeper than pastel.
Although the microbubbles disclosed in the U.S. Pat. No. 4,223,067 examples are glass, they can be polymeric of the type described in U.S. Pat. No. 4,615,472 or U.S. Pat. No. 4,287,308. Because these polymeric microbubbles are smaller and less dense than commercially available glass microbubbles, a larger number is required to attain the same volume loading of microbubbles. Hence, pigmented foamlike tapes tend to be even lighter colored when filled with polymeric microbubbles than when filled with glass microbubbles.
Also useful for purposes previously requiring a foam-backed pressure-sensitive adhesive tape is the cellular pressure-sensitive adhesive tape disclosed in U.S. Pat. No. 4,415,615. It preferably is made by polymerizing a coated froth in situ by exposure to ultraviolet radiation. Although the use of pigments is suggested (col. 5, lines 48-57), the ultraviolet radiation would be blocked if the froth contained appreciable amounts of carbon black or other pigment.
The above-cited U.S. Pat. No. 4,223,067 also teaches that where it is desired to adhere the microbubble-filled tape to a surface to which its pressure-sensitive adhesive layer would not form a strong bond, it may be desirable to apply to one or both of its faces a layer of unfilled pressure-sensitive adhesive which is especially selected for its adhesion to that surface (col. 4, lines 9-15). The above-cited U.S. Pat. No. 4,416,615 contains a similar teaching (col. 5, lines 3-9).